The present invention relates to a workpiece centering apparatus and a method of centering a workpiece.
Various types of polishing apparatuses for polishing workpieces have been known.
Each of the polishing apparatuses has a polishing plate, in which polishing cloth is adhered on an upper face, and a top ring pressing a workpiece, which is held on the lower side, onto the polishing cloth. The polishing plate is relatively moved with respect to the top ring so as to polish a lower face of the workpiece.
The workpiece, which has been polished by the polishing apparatus, is put into a tray storing water and washed therein, and then the workpiece is transferred to a next stage, e.g., accommodating cassette, by a holding unit, e.g., robot hand. To hold the workpiece by the holding unit, the workpiece must be centered.
A conventional workpiece centering apparatus is disclosed in Japanese Patent Gazette No. 2003-62750. In the workpiece centering apparatus, water is jetted toward an upper face of a tray so as to form water film, a polished workpiece, which is held on a lower face of a chucking mechanism, is dropped into the tray with jetting water, the polished surface of the workpiece is cleaned by the jetted water, a cylinder rod of an elevating unit is extended upward so as to center the workpiece by picots of a positioning mechanism, the workpiece, whose outer edge has been fixed by the picots, is moved upward until reaching a position above a fringe section of the tray, which is upwardly extended from an outer edge of the tray, and an end part is clamped by a robot arm and transferred to an accommodating cassette.
However, in the conventional workpiece centering apparatus, the workpiece is dropped into the tray and washed, and then the tray is moved upward by the picots, whose upper end parts are formed into tapered shapes, and compulsorily positioned by the tapered parts. Therefore, the picots moving upward apply shocks to the workpiece, so that the workpiece will be damaged. Namely, when the picots are moved upward, a shifted end part of the workpiece, which is not centered, strongly collides with the tapered part of the picot and moved downward along the tapered part, and the other end part of the workpiece collides with the picot, so that the workpiece is centered. With this action, the strong shock is applied to the workpiece by the picots.